With an increase of integration of IC circuits, heat value is increased in electronic parts such as hybrid packages, multimodules, and sealed-type integrated circuits with plastics and metals. Since the increase of temperature due to the increased heat value may cause malfunction of the electronic parts, a countermeasure for preventing the malfunction by attaching a heat-radiating member such as a heat sink to the electronic part has been taken.
As a method for providing electronic parts with heat-radiating members, a method which comprises using an adhesive prepared by adding aluminum powder etc. to a composition comprising a polymerizable acrylate monomer and a free radical initiator is proposed in U.S. Pat. No. 4,722,960. In this method, after the adhesive is applied onto one or both of the electronic part and the heat-radiating member, it is necessary to effect curing treatment using a primer or blocking oxygen. Such an adhesion treatment needs a long time and much labor, and the materials to be adhered have to be fixed temporally until the completion of curing, thereby resulting in poor efficiency in producing the electronic devices.
Further, U.S. Pat. No. 4,606,962 proposes to use adhesive tape which has an adhesive layer containing silver grains having a diameter exceeding the thickness of the layer. When silver grains are added in the process of the preparation of the adhesive composition, the viscosity of the composition is extremely increased and, as a result, the composition becomes poor in flowability and handling properties, in particular, coating workability. When such a composition is formed into a tape, no thickness accuracy can be achieved, and in the worst case, no tape can be obtained.
Also, JP-A-6-88061 corresponding to EP 566093 A1 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method with the use of heat-conductive and electrical insulating tapes comprising alkyl acrylate polymers containing polar monomers in which heat-conductive and electrical insulating grains are dispersed at random. With the recent increase of integration of member packaging, high electrical insulating properties are needed in such tapes for electronic parts. However, the above-mentioned heat-conductive and electrical insulating tapes have no base material, which makes it impossible to regulate pinholes in the sheets. Moreover, it is feared that the terminal of an electronic part might penetrate into the pressure-sensitive adhesive and thus seriously deteriorate the electrical insulating properties (volume resistivity, breakdown voltage). Therefore, the above-mentioned heat-conductive and electrical insulating tapes are unusable in sites where a high reliability is needed.
On the other hand, as a heat-conductive interfacial material located between a heat source and a radiator, JP-A-5-198709 (U.S. Pat. No. 5,213,868) proposes a heat-conductive material comprising, as a base material, a plastic film (made of nylon, polyesters, polyamides, polyimides, etc.) optionally containing aluminum oxide or boron nitride and a layer of a pressure-sensitive acrylic adhesive containing a heat-conductive filler formed thereon. The material is perforated, embossed or grooved so as to remove the air between the heat source and the radiator. However, these materials are poor in the adhesion between the base material and the pressure-sensitive adhesive layer and suffer from a problem of peeling between these members when used at high temperatures, i.e., anchoring fracture.
It is frequently attempted in the art to form one or more undercoating layers between a base material and a pressure-sensitive adhesive layer so as to improve the adhesion between them. This treatment comprises using an undercoating material having a polarity between those of the base material and the pressure-sensitive layer, dissolving the material in, for example, an organic solvent, and then applying it onto the base material followed by drying. However, the application of this method to the above-mentioned heat-conductive materials, etc. suffers from some troubles such as a need for complicated operations and worsened working environment due to the use of the organic solvent. It is also known to improve the adhesion by a dry system such as corona discharge treatment or sputtering. However, such a system is not available in practice, since it can achieve only insufficient effects or the thus achieved effects are deteriorated with the passage of time.
Considering the above-mentioned problems encountering in the prior arts, the present invention aims at providing heat-conductive and pressure-sensitive adhesive sheets which are to be used in, for example, fixing electronic parts to heat-radiating members and can achieve the above-mentioned objects of fixing while needing neither a long time nor much labor in adhesion. The heat-conductive and pressure-sensitive adhesive sheets of the present invention, which are composed of a base material and a pressure-sensitive adhesive composition layer formed thereon, are excellent in pressure-sensitive adhesion and heat conductivity, result in no troubles such as complicated operations or worsened working environment during the production thereof, show excellent adhesion between the base material and the pressure-sensitive adhesive composition layer, scarcely undergo anchoring fracture between them even after allowing to stand at high temperatures for a long time and have excellent heat resistance.
In order to achieve the above-mentioned object, the present inventors have conducted extensive studies and, as a result, have achieved the present invention based on the finding that heat-conductive and pressure-sensitive adhesive sheets having excellent pressure-sensitive adhesion and heat conductivity, showing improved adhesion of the adhesive layer to the base material, scarcely undergoing anchoring fracture between the base material and the adhesive layer even after allowing to stand at high temperatures for a long time, and having a high heat resistance can be obtained, without resort to the above-mentioned undercoating which causes troubles such as complicated operations or worsened working environment, by forming a specific thin layer on a base material made of an electrical insulating plastic film and then forming a pressure-sensitive adhesive composition layer thereon.